JP6741861B2 - Ferritic stainless steel pipe with excellent salt damage resistance in the gap, pipe end thickening structure, welded joint, and welded structure - Google Patents

Description

The present invention relates to a ferritic stainless steel pipe, a pipe end thickened structure, and a welded joint, which require corrosion resistance in a gap structure portion.
The present application claims priority based on Japanese Patent Application No. 2017-069284 filed in Japan on March 30, 2017, and the content thereof is incorporated herein.

Ferritic stainless steel is used in a wide range of fields such as home appliances, electronic devices, and automobiles. Particularly in the automobile field, since it is used in various parts such as an exhaust manifold and a muffler, the stainless steel used is required to have heat resistance and corrosion resistance. In addition, since these parts are welded in most cases, strength, rigidity and corrosion resistance of the welded part are also required.

In recent years, in order to reduce the weight of automobiles, there are increasing cases where thinning of materials used for each component is considered. However, in order to secure the strength, rigidity, and weldability of the welded portion, a certain thickness may be required, and even the non-welded portion becomes thicker, which hinders the exhaust system from becoming thinner. On the other hand, there is known a technique of increasing the thickness of the welded portion by increasing the thickness of the end portion of the steel pipe that constitutes the exhaust pipe and is joined to other parts by welding, thereby increasing the strength and ensuring rigidity and weldability. ing. This is called pipe end thickening (thickening the pipe end portion of the steel pipe). In this case, the non-welded portion can be made thin, and the exhaust system as a whole can be made thin and lightweight.

Several techniques regarding the above-described pipe end thickening are disclosed. In Patent Document 1, for the purpose of securing the strength of the pipe end and reducing the weight of the pipe, a roller is pressed against the end while rotating the pipe to bend the pipe inward in the radial direction, and then a process of closely contacting with the roller. A method is disclosed. Patent Document 2 discloses a method for forming a pipe end into a double pipe shape and doubling the wall thickness to prevent burn-through during welding. Patent Document 3 discloses a patent relating to a raw pipe for folding back the pipe end to increase the wall thickness, the inner surface bead portion of the welded portion is projected to the inner surface of the pipe, and the projection amount is 4 to 15 of the plate thickness. It is prescribed as%.

The pipes with thickened pipe ends described in Patent Documents 1 to 3 have a gap structure with a height of several μm to several hundreds μm at the bent portion. When the gap portion is bent inward as in Patent Documents 1 and 2, exhaust gas condensed water generated inside the exhaust system component is likely to stay in the gap portion. When bent outward as in Patent Document 3, salt water adhering from the outside of the exhaust system component tends to stay in the gap.
Corrosion that occurs in this environment is not crevice corrosion, but salt damage corrosion that is promoted by the tendency that salt water and exhaust gas condensed water easily accumulate in the crevice environment. Since the corrosion in the gap may be accelerated as described above, the stainless steel used is required to be a steel type having excellent salt damage resistance in the gap. Especially in exhaust system parts, since perforation due to corrosion leads to leakage of exhaust gas, it is important to apply a material having high perforation resistance.

In patent document 4, C: 0.001-0.02%, N: 0.001-0.02%, Si: 0.01-0.5%, Mn: 0.05-1 in mass%. %, P: 0.04% or less, S: 0.01% or less, Cr: 12 to 25%, Ti: 0.02 to 0.5% and Nb: 0.02 to 1%. Disclosed is a ferritic stainless steel excellent in crevice corrosion resistance, characterized in that it contains either or both of them, and further contains Sn: 0.005 to 2% and the balance is Fe and unavoidable impurities. .. The technique described in Patent Document 4 improves crevice corrosion resistance by adding Sn, but does not describe the relationship with the gap spacing.

In Patent Document 5, C: ≤ 0.015%, Si: 0.10 to 0.50%, Mn: 0.05 to 0.50%, P ≤ 0.050%, S: ≤ in mass%. 0.0100%, N: ≤ 0.015%, Al: 0.020 to 0.100%, Cr: 10.5-13.05%, and Ti: 0.03 to 0.30%. And Nb: 0.03 to 0.30%, either or both, Sn: 0.03 to 0.50%, and Sb: 0.03 to 0.50%, either or both. The balance is Fe and inevitable impurities, and the A value defined by the formula (2) is 15.23 or more, which is excellent in corrosion resistance after heating and is an alloy-saving ferrite for automobile exhaust system members. Series stainless steels are disclosed.
A=[Cr]+[Si]+0.5[Mn]+10[Al]+15([Sn]+[Sb]) Equation (2)
The technique described in Patent Document 5 improves the corrosion resistance after heating by adding Sn and Sb, but does not describe the corrosion resistance when a gap is present.

In Patent Document 6, C: ≤ 0.015%, Si: 0.01 to 0.50%, Mn: 0.01 to 0.50%, P ≤ 0.050%, and S: ≤ by mass%. 0.010%, N: ≤ 0.015%, Al: 0.010 to 0.100%, Cr: 16.5 to 22.5%, and Ti: 0.03 to 0.30%. And Nb: 0.03 to 0.30%, either or both, and Sn: 0.05 to 1.00%, with the balance being Fe and inevitable impurities. , A Mo-saving ferritic stainless steel for an automobile exhaust system member having excellent corrosion resistance after heating is disclosed. The technique described in Patent Document 6 improves the corrosion resistance after heating by adding Sn, but does not describe the corrosion resistance when a gap is present.

In Patent Document 7, C:≦0.015%, Si:0.01 to 0.50%, Mn:0.01 to 0.50%, P≦0.050%, S:≦% by mass. 0.010%, N: ≤ 0.015%, Al: 0.010 to 0.100%, Cr: 16.5 to 22.5%, Ni: 0.5 to 2.0%, Sn: 0. 01 to 0.50%, further contains either or both of Ti: 0.03 to 0.30% and Nb: 0.03 to 0.30%, the balance being Fe and unavoidable. Disclosed is a ferritic stainless steel for automobile exhaust system members, which is characterized by comprising impurities. The technique described in Patent Document 7 discloses the corrosion resistance of exhaust system components after heating, but does not describe the corrosion resistance in a gap environment.

In Patent Document 8, C: 0.0150% or less, Si: 1.0 to 1.5%, Mn: 0.15 to 1.0%, P: 0.050% or less, and S: in mass%. 0.0100% or less, N: 0.0150% or less, Al: 0.010 to 0.200%, Cr: 13.0 to 16.0%, and Sn: 0.002 to 0.050% are contained. In addition, one or both of Ti: 0.03 to 0.30% and Nb: 0.03 to 0.50% are contained, and the A value defined by the formula (1) is 0.024 or more. Disclosed is a ferritic stainless steel for automobile exhaust system members, which is excellent in oxidation resistance and corrosion resistance and is characterized in that the balance thereof is Fe and inevitable impurities.
A=[Si]×[Sn]+0.014[Si] (1)
Here, [Si] and [Sn] are the contents of Si and Sn as mass%, respectively.
The technique described in Patent Document 8 discloses the corrosion resistance of exhaust system components after heating, but does not describe the corrosion resistance in a gap environment.

In Patent Document 9, C: 0.0150% or less, Si: 0.2 to 0.7%, Mn: 0.2 to 0.6%, P: 0.050% or less, and S: in mass%. 0.0100% or less, N: 0.0150% or less, Al: 0.010 to 0.20%, Cr: 10.5-11.5%, Mo: 0.02 to 0.20%, and Sn: 0.005 to 0.050%, and either or both of Ti: 0.03 to 0.30% and Nb: 0.03 to 0.50%, and the following (1 ), a ferritic stainless steel for exhaust system members excellent in corrosion resistance, characterized in that the A value defined by the formula is 0.00065% ² or more, and the balance is Fe and unavoidable impurities. ing.
A=[Mo]×[Sn] (1)
The technique described in Patent Document 9 discloses the corrosion resistance of exhaust system components after heating, but does not describe the corrosion resistance in a gap environment.

As described above, in the prior art, no method has yet been proposed for improving the corrosion resistance in the gap environment formed at the pipe end portion of the pipe having the increased pipe end thickness.

JP, 2010-234406, A JP, 2013-103250, A JP, 2004-255414, A Japanese Patent No. 4727601 Japanese Patent No. 5297713 Japanese Patent No. 5320034 Patent No. 5586279 Japanese Patent No. 6006660 JP, 2014-169491, A

The present invention provides a solution for improving the corrosion resistance of the crevice environment formed at the pipe end portion of the pipe having the increased pipe end thickness.

In order to solve the above-mentioned problems, the inventors of the present invention have earnestly studied the corrosion resistance of the ferritic stainless steel pipe gap. As a result, it was found that in a crevice environment, the higher the Cr content, the higher the pitting depth. Then, it was found that there is a relationship between the Cr amount and the Sn amount and the critical gap distance at which pitting corrosion deeply grows.

Means for solving the above problems have the following configurations.
[1] The ferritic stainless steel pipe excellent in salt damage resistance of the gap portion according to one aspect of the present invention is C: 0.001 to 0.100%,
Si: 0.01 to 2.00%,
Mn: 0.01 to 2.00%,
P: 0.001-0.05%,
S: 0.0001 to 0.005%,
Cr: 10.5 to 20.0%,
Sn: 0.001 to 0.600%,
Ti: 0.001-1.000%,
Al: 0.001 to 0.100%,
N: 0.001 to 0.02% is contained, the balance is Fe and unavoidable impurities, and a pipe end thickened portion is provided at the pipe end, and a gap distance d (μm ) Satisfies the relationship of d≧Cr ² /(1000Sn) (Cr and Sn in the formula represent the content (mass %) of each element).

[2] Further, in mass%, Ni: 0.1 to 1.0%,
Mo: 0.1-3.0%,
Cu: 0.10 to 3.00%,
B: 0.0001 to 0.0050%,
Nb: 0.001 to 0.300%,
W: 0.001-1.00%,
V: 0.001 to 0.50%,
Sb: 0.001 to 0.100%,
Co: 0.001 to 0.500%,
The ferritic stainless steel pipe having excellent salt damage resistance in the gap according to the above [1], which contains one or more of any of the above.

[3] Further, in mass% Ca: 0.0001 to 0.0050%,
Mg: 0.0001 to 0.0050%,
Zr: 0.0001 to 0.0300%,
Ga: 0.0001 to 0.0100%,
Ta: 0.001 to 0.050%,
REM: 0.001 to 0.100%,
The ferritic stainless steel pipe having excellent salt damage resistance in the gap portion according to the above [1] or [2], characterized in that it contains any one kind or two or more kinds thereof.
[4] A ferritic stainless steel pipe having excellent salt damage resistance in the gap portion according to any one of [1] to [3], which is used in a pipe end thickened structure.

[5] A pipe end thickened structure according to an aspect of the present invention is characterized by being formed of the ferritic stainless steel pipe according to any one of [1] to [4].
[6] A welded joint according to one aspect of the present invention is characterized by having a pipe end thickened portion made of the ferritic stainless steel pipe according to any one of the above [1] to [4].
[7] If the plate thickness of the single pipe portion of the ferritic stainless steel pipe is t, further having a structure joined by welding to the pipe end thickened portion, then the ferritic stainless steel pipe side of the welded portion Has a maximum penetration depth of 0.3 t to 2.0 t, and the welding joint according to [6] above.
[8] A welded structure according to an aspect of the present invention is characterized by having the welding joint described in [7] above.

According to one aspect of the present invention, a ferritic stainless steel pipe excellent in crevice corrosion resistance (crevice corrosion resistance), a pipe end thickened structure including the ferritic stainless steel pipe, a welded joint having a pipe end thickened portion, And a welded structure having a welded joint can be provided.

It is sectional drawing which shows the joint structure of the pipe end thickening pipe which consists of stainless steel pipes, and another steel pipe. FIG. 2 is an enlarged view of the periphery of the welded portion 3 in FIG. 1, where (a) shows the case where the maximum penetration depth is 0.3 t, where t is the plate thickness of the single pipe portion of the ferritic stainless steel pipe, ) Shows the case where the maximum penetration depth is 1.0t, (c) shows the case where the maximum penetration depth is 2.0t, and (d) shows the case where the maximum penetration depth exceeds 2.0t. Indicates. It is a graph which shows the relationship between the amount of Cr and the amount of Sn of a pipe end thickening pipe manufactured in an example, and a critical gap interval.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
In order to simulate the gap environment of the pipe end thickened pipe (ferritic stainless steel pipe) and evaluate the corrosion resistance, the present inventors produced steel sheets of various compositions. Then, test pieces having various gap intervals simulating the gap portion of the pipe end thickened pipe were produced from these steel plates by spot welding. The corrosion test was performed 100 cycles according to the corrosion test method of the appearance of automobile parts of JASO-M610-92, and the salt damage corrosion resistance of the gap was evaluated. The maximum pitting depth was used for evaluation, and samples with a maximum pitting depth of less than 500 μm were evaluated as “good”, and samples with a maximum pitting depth of 500 μm or more were evaluated as “x” (poor). evaluated.

FIG. 3 shows the relationship between the amount of Cr in the base material in various amounts of Sn in the base material and the critical gap distance at which corrosion in the gap portion is suppressed based on the above-described test results. Although Cr in the base material improves the corrosion resistance in a general environment, it can be seen from FIG. 3 that the pitting depth in the crevice environment increases as the amount of Cr in the base material increases. It was also found that the critical gap spacing becomes smaller by increasing the amount of Sn added to the steel sheet (as the amount of Sn in the base metal increases).

By observing the corrosion morphology of the crevices of steel types with a high Cr content, it was found that a small number of pitting corrosion deeply grew. On the other hand, in the corrosion morphology of the crevice of the low Cr content steel type, many pitting corrosions occurred, but it was found that the depth of each pitting corrosion is shallower than that of the high Cr content steel type. ..

It is considered that the steel type with a high Cr content has a high Cr concentration in the passivation film and high corrosion resistance, and therefore the number of pitting corrosions decreased. Therefore, it is considered that the oxygen reduction reaction, which is a cathode reaction, contributes only to the growth of a small number of pits, and that each pit is deeply grown. On the other hand, it is considered that, in the steel type with a low Cr content, the cathodic reaction contributes to the generation of a large number of pitting corrosions, so that the pitting depths of individual steels did not grow significantly.

Further, it was found from the above-mentioned test that Sn has an effect on the occurrence of pitting corrosion in a crevice environment. It has been known that Sn suppresses active dissolution of stainless steel and improves crevice corrosion resistance. However, the fact that Sn suppresses the occurrence of pitting corrosion in the crevice environment and reduces the critical crevice spacing is a new finding based on the test results of this time.

(Ferrite stainless steel pipe)
The chemical composition of steel defined in this embodiment will be described in more detail below. In addition,% means mass %.
C: C lowers the intergranular corrosion resistance and workability of steel, so its content must be kept low. Therefore, the C content is set to 0.100% or less. However, an excessively low content raises the scouring cost, and therefore the C content is preferably 0.001% or more. The C content is more preferably 0.003 to 0.050%, further preferably 0.005 to 0.020%.

Si:Si is useful as a deoxidizing element, but if added in excess, it hardens the material, so the content is made 0.01 to 2.00%. The Si content is more preferably 0.02 to 0.80%, further preferably 0.03 to 0.70%.

Mn: Mn is useful as a deoxidizing element, but if it is contained excessively, the corrosion resistance deteriorates, so the content is made 0.01 to 2.00%. The Mn content is more preferably 0.02 to 0.80%, further preferably 0.03 to 0.70%.

P: P is an element that deteriorates workability, weldability, and corrosion resistance, and its content needs to be limited. Therefore, the P content is set to 0.05% or less. However, excessively reducing the amount of P increases the refining cost, so the lower limit value is made 0.001%. The P content is more preferably 0.003 to 0.04%, further preferably 0.005 to 0.03%.

S: S is an element that deteriorates corrosion resistance, so its content must be limited. Therefore, the S content is set to 0.005% or less. However, if the amount of S is excessively reduced, the refining cost increases, so the lower limit value is made 0.0001%. The S content is more preferably 0.0003 to 0.003%, further preferably 0.0005 to 0.001%.

Cr: Cr is necessary in an amount of at least 10.5% or more in order to secure salt corrosion resistance and exhaust gas condensed water corrosion resistance. Although the corrosion resistance is improved as the Cr content is increased, the workability and manufacturability are lowered, and the cost is increased. Therefore, the upper limit is set to 20.0% or less. The Cr content is more preferably 11.0 to 19.0%, further preferably 13.0 to 17.5%.

Sn:Sn is an element that improves the corrosion resistance, and is required to be at least 0.001% or more. The Sn content of 0.001 to 0.009% is effective, but increasing the Sn content is more effective. However, excessive addition lowers workability and manufacturability, so the upper limit is made 0.600% or less. The Sn content is more preferably 0.002 to 0.500%, further preferably 0.030 to 0.300%. Considering the cost, 0.030 to 0.100% is desirable.

Ti: Ti is an element useful for improving the corrosion resistance and can be contained in an amount of 0.001% or more. However, if added excessively, the cost will increase, so the upper limit is made 1.000%. The Ti content is more preferably 0.002 to 0.500%, further preferably 0.003 to 0.200%.

Al: Al is an element useful for refining such as deoxidizing effect, and is required to be at least 0.001%. However, if added excessively, coarse inclusions are formed and the corrosion resistance is reduced, so the upper limit is made 0.100% or less. The Al content is more preferably 0.005 to 0.080%, further preferably 0.010 to 0.070%.

N: N deteriorates formability and corrosion resistance, so the N content is made 0.02% or less. However, excessive reduction leads to an increase in refining cost, so the lower limit is made 0.001%. The N content is more preferably 0.002 to 0.015%, further preferably 0.003 to 0.010%.

The above is the basic chemical composition of the ferritic stainless steel of the present embodiment, but in the present embodiment, the following elements can be further contained as necessary.
Ni: Ni is an element useful for improving the corrosion resistance, and can be contained in an amount of 0.1% or more. However, excessive addition increases cost, so the upper limit is made 1.0%. The Ni content is more preferably 0.2 to 0.8%, further preferably 0.3 to 0.5%.

Mo: Mo is an element useful for improving the corrosion resistance and can be contained in an amount of 0.1% or more. However, excessive addition increases cost, so the upper limit is made 3.0%. The Mo content is more preferably 0.2 to 2.0%, further preferably 0.3 to 1.5%.

Cu: Cu is an element useful for improving the corrosion resistance and can be contained in an amount of 0.10% or more. However, excessive addition increases cost, so the upper limit is made 3.00%. The Cu content is more preferably 0.20 to 2.00%, further preferably 0.30 to 1.50%.

B: B is an element useful for improving hot workability and can be contained in an amount of 0.0001% or more. However, excessive addition lowers the corrosion resistance, so the upper limit is made 0.0050% or less. The B content is more preferably 0.0005 to 0.0030%, further preferably 0.0010 to 0.0010%.

Nb: Nb is an element useful for improving the corrosion resistance, and it is desirable to contain 0.001% or more. However, excessive addition deteriorates workability and manufacturability, so the upper limit is made 0.300% or less. The Nb content is more preferably 0.005 to 0.200%, further preferably 0.010 to 0.100%.

W: W is an element useful for improving corrosion resistance, and is preferably contained in an amount of 0.001% or more. However, excessive addition lowers workability and manufacturability, so the upper limit is made 1.00% or less. The W content is more preferably 0.005 to 0.70%, further preferably 0.010 to 0.50%.

V: V is an element useful for improving the corrosion resistance, and is preferably contained in an amount of 0.001% or more. However, excessive addition lowers workability and manufacturability, so the upper limit is made 0.50% or less. The V content is more preferably 0.005 to 0.40%, further preferably 0.010 to 0.30%.

Sb: Sb is an element useful for improving corrosion resistance, and is preferably contained in an amount of 0.001% or more. However, excessive addition deteriorates workability and manufacturability, so the upper limit is made 0.100% or less. The Sb content is more preferably 0.005 to 0.080%, further preferably 0.010 to 0.050%.

Co: Co is preferably contained in an amount of 0.001% or more in order to improve secondary workability and toughness. However, excessive addition lowers workability and manufacturability, so the upper limit is made 0.500% or less. The Co content is more preferably 0.002 to 0.400%, further preferably 0.003 to 0.300%.
The total content of one or more of Ni, Mo, Cu, B, Nb, W, V, Sb, and Co is preferably 6% or less from the viewpoint of cost increase.

Ca: Ca is preferably contained in an amount of 0.0001% or more in order to improve desulfurization and hot workability. However, if added excessively, water-soluble inclusions CaS are formed and corrosion resistance is reduced, so the upper limit is made 0.0050%. The Ca content is more preferably 0.0002 to 0.0045%, further preferably 0.0003 to 0.0040%.

Mg: Mg is preferably contained in an amount of 0.0001% or more in order to refine the structure and improve workability and toughness. However, if added excessively, the hot workability is deteriorated, so the upper limit is made 0.0050%. The Mg content is more preferably 0.0003 to 0.0040%, further preferably 0.0005 to 0.0030%.

Zr: Zr is preferably contained in an amount of 0.0001% or more in order to improve the corrosion resistance. However, excessive addition deteriorates workability and manufacturability, so the upper limit is made 0.0300%. The Zr content is more preferably 0.0005 to 0.0200%, further preferably 0.0010 to 0.0100%.

Ga: Ga is preferably contained in an amount of 0.0001% or more in order to improve the corrosion resistance and the hydrogen embrittlement resistance. However, excessive addition deteriorates workability and manufacturability, so the upper limit is made 0.0100%. The Ga content is more preferably 0.0005 to 0.0080%, further preferably 0.0010 to 0.0050%.

Ta: Ta is preferably contained in an amount of 0.001% or more in order to improve the corrosion resistance. However, excessive addition lowers workability and manufacturability, so the upper limit is made 0.050%. The Ta content is more preferably 0.005 to 0.040%, further preferably 0.010 to 0.030%.

REM: REM is a useful element for refining because it has a deoxidizing effect and the like, and is preferably contained in an amount of 0.001% or more. However, excessive addition lowers workability and manufacturability, so the upper limit is made 0.100%. The REM content is more preferably 0.005 to 0.080%, further preferably 0.010 to 0.050%.
REM is a rare earth metal such as Ce, La, Pr, Nd. "REM content" means the total value of the content of all these REM elements. If the total content is within the above range, the same effect can be obtained whether the REM element is one kind or two or more kinds.

The ferritic stainless steel pipe of this embodiment includes a pipe end thickened portion 1a at the pipe end as shown in FIG. The pipe end thickened portion 1a refers to a portion where the thickness is increased at the pipe end portion of the steel pipe. The pipe end thickened portion 1a is formed, for example, by folding an end portion of a steel pipe inward or outward by 180°. Therefore, the pipe end thickened portion 1a has an end portion that is folded back inward or outward. In the pipe end thickened portion 1a, a gap portion 1b exists between the outer portion (outer peripheral portion) and the inner portion (inner peripheral portion) of the stainless steel pipe. That is, in the pipe end portion of the steel pipe, the gap portion 1b exists between the steel pipe and the folded back portion of the steel pipe. The maximum value of the gap distance between the steel pipe and the folded back portion of the steel pipe is called the gap distance d (μm).
The gap distance d (μm) existing at the tube end satisfies the relationship of d≧Cr ² /(1000Sn) (Cr and Sn in the formula represent the content (mass %) of each element).

The pipe end thickened pipe (ferritic stainless steel pipe) of the present embodiment is made of a stainless steel plate having a steel component specified in the present embodiment. The method for manufacturing the stainless steel plate is steelmaking-hot rolling-annealing. The process consists of pickling-cold rolling-annealing, and the manufacturing conditions of each process are not particularly specified.
In steelmaking, a method in which steel containing the above-mentioned essential components and components added as necessary is melted in a converter and then secondary refining is performed is preferable. The molten steel produced is made into a slab according to a known casting method (continuous casting). The slab is heated to a predetermined temperature and hot-rolled to a predetermined plate thickness by continuous rolling. The annealing step after hot rolling may be omitted, and in the cold rolling after pickling, it may be rolled with either a normal Sendzimir mill or a tandem mill, but considering the bendability of the steel pipe, tandem mill rolling Is preferable.

In cold rolling, roll roughness, roll diameter, rolling oil, number of rolling passes, rolling speed, rolling temperature and the like may be appropriately selected within a general range. Intermediate annealing may be performed during cold rolling, and the intermediate and final annealing may be batch type annealing or continuous type annealing. Regarding the annealing atmosphere, if necessary, bright annealing may be performed in a non-oxidizing atmosphere such as hydrogen gas or nitrogen gas, or annealing may be performed in the air. Further, the product plate may be subjected to lubrication coating to further improve press molding, and the type of the lubricating film may be appropriately selected. After the final annealing, temper rolling or a leveler may be added to correct the shape, but it is desirable not to add these because it causes a decrease in work hardening ability.

The method for manufacturing the steel pipe may be appropriately selected and is not limited to the welding method, and ERW (resistance welding), laser welding, TIG welding (tungsten inert gas welding) or the like may be appropriately selected. Also, the size of the steel pipe may be determined according to the application. The pipe end thickening process from a steel pipe is preferably a pipe end spinning process or a forging process, but these process methods are not particularly specified.
In addition, there are cases where the pipe is formed outside the pipe and the inside of the pipe is formed. When the pipe is formed outside the pipe, the inside diameter of the formed portion is the same as that of the raw pipe. On the other hand, when the inside of the pipe is to be machined, the outer diameter of the machined portion is the same as that of the raw pipe. Taking work efficiency and dimensional accuracy into consideration, spinning is preferable, and it is preferable to adopt a method in which the pipe end is bent once and brought into close contact in the next step.

When the gap distance d formed at the pipe end portion satisfies the above relational expression, it is possible to realize a ferritic stainless steel pipe capable of providing a pipe end thickened structure excellent in salt corrosion resistance of the gap portion.
By using a ferritic stainless steel pipe satisfying the above-mentioned composition and relational expression as automobile parts and motorcycle parts in particular, it is possible to reduce the wall thickness, and to efficiently manufacture parts and improve the fuel efficiency of automobiles and motorcycles to which they are applied. It will be possible.
Further, according to the ferritic stainless steel pipe satisfying the above-described composition and relational expression, it is possible to provide a welded joint and a welded structure having a pipe end thickened portion having excellent salt corrosion resistance in the gap portion.

(Pipe end thickened structure, welded joint, welded structure)
As described above, the ferritic stainless steel pipe of the present embodiment has a composition containing C, Si, Mn, P, S, Cr, Sn, Ti, Al and N in an amount within a specified range, and It is a stainless steel pipe for a pipe-end thickening structure that is to be machined or machined inside the pipe.
The pipe end thickened structure of the present embodiment has the ferritic stainless steel pipe of the present embodiment. The pipe end thickened structure refers to a structure having a steel pipe and a pipe end thickened portion provided on the steel pipe. In the present embodiment, the gap distance d (μm) formed at the tube end portion is d≧Cr ² /(1000Sn) (Cr and Sn in the formula represent the content (mass %) of each element). A pipe end thickening structure having a relationship can be provided.
This pipe end thickened structure has a feature that it is excellent in salt damage resistance in the gap.

The welded joint of the present embodiment has a pipe end thickened portion made of the ferritic stainless steel pipe of the present embodiment. That is, this welded joint has the pipe end thickened portion of the ferritic stainless steel pipe of the present embodiment. In other words, the welded joint of the present embodiment has the ferritic stainless steel pipe of the present embodiment, and this pipe has a pipe end thickened portion.
The welded joint according to the present embodiment has a pipe end thickened portion having excellent resistance to salt damage in the gap.
By using the welded joint of the present embodiment, in particular, as automobile parts and motorcycle parts, it is possible to reduce the thickness of the parts, and it is possible to efficiently manufacture the parts and improve the fuel efficiency of automobiles and motorcycles to which the parts are applied.

FIG. 1 shows a joint A in which another steel pipe 2 is joined to the pipe end thickened structure 1 made of the above-mentioned ferritic stainless steel pipe by welding.
A pipe end thickened portion 1a is formed by providing an inwardly folded portion at the pipe end portion of the pipe end thickened structure 1. That is, the pipe end thickened portion 1a in FIG. 1 is formed by folding the end portion of the steel pipe inward by 180°. The steel pipe 2 is joined to the outside of the pipe end thickened portion 1a by a welded portion 3.
A gap portion 1b is formed between the outer portion and the inner portion of the stainless steel pipe in the pipe end thickened portion 1a.
In the joint A having the structure shown in FIG. 1, a gap portion 1b having a gap distance d satisfying the above relational expression is formed in accordance with the above composition of the ferritic stainless steel pipe. This makes it possible to obtain excellent resistance to salt damage in the gaps.

When welding the pipe end thickened pipe (ferritic stainless steel pipe) to the structure, any welding method may be adopted in the same manner as above. Examples of the structure include a steel pipe.
Single pipe of ferritic stainless steel pipe when joining the structure to the thickened part of the ferritic stainless steel pipe by welding (when the welded joint further has a structure joined to the thickened part of the pipe end by welding) When the plate thickness of the portion is t, the maximum penetration depth on the ferritic stainless steel pipe side of the welded portion is preferably 0.3t to 2.0t.
The maximum penetration depth is measured by the following method. The cross section of the welded portion is observed, and the deepest melted portion of the welded portion is defined as the maximum penetration portion, and the depth is defined as the maximum penetration depth.
FIG. 2 shows an enlarged view around the welded portion 3 of FIG. Assuming that the plate thickness of the single pipe portion of the ferritic stainless steel pipe is t, FIG. 2(a) shows a case where the maximum penetration depth is 0.3t, and FIG. 2(b) shows a maximum penetration depth of 1t. 2C shows the case where the maximum penetration depth is 2.0t, and FIG. 2D shows the case where the maximum penetration depth is more than 2.0t.
1 and 2 show the case where the welded portion 3 is formed by bringing the electrode/arc close to the outer peripheral surface side of the pipe end thickened portion and performing welding. Therefore, the outer peripheral surface of the pipe end thickened portion becomes the electrode/arc side surface, and the inner peripheral surface of the pipe end thickened portion becomes the surface (rear surface) opposite to the electrode/arc side surface. The maximum penetration depth is the distance (depth) from the outer peripheral surface of the thickened pipe end to the maximum penetration.
As shown in FIG. 2, when the maximum penetration portion does not reach the inner peripheral surface of the pipe end thickened portion, the maximum penetration depth is less than 2.0t. When the maximum penetration portion has just reached the inner peripheral surface of the pipe end thickened portion, the maximum penetration depth is 2.0 t. When the maximum penetration portion reaches the inner peripheral surface of the pipe end thickened portion and the melted portion also exists on the inner peripheral surface, the maximum penetration depth is more than 2.0 t. That is, the case where the maximum penetration depth exceeds 2.0 t is the case where the melted portion exists on the surface (rear surface) opposite to the surface on the electrode/arc side during welding.
By setting the maximum penetration depth to 0.3 t or more, the strength of the welded portion is secured (secured), and a welded joint having a pipe end thickened portion with excellent salt corrosion resistance in the gap portion and welding described later. A structure is obtained. When the maximum penetration depth exceeds 2.0 t, the shape of the welded portion becomes uneven, which may lead to various problems such as reduction in strength, deterioration in corrosion resistance, and leakage of exhaust gas.

The reason why a welded joint having a pipe end thickened portion having excellent salt corrosion resistance in the gap portion can be obtained is shown below.
By setting the maximum penetration depth to 0.3 t or more, the shape of the welded portion on the outer side of the pipe end thickened pipe (ferritic stainless steel pipe) is stabilized, and a gap structure that may be a starting point of corrosion is not formed. The maximum penetration depth is preferably more than 1.0 t, and in this case, the inner gap of the pipe end thickened pipe (ferrite stainless steel pipe) is also closed, and the gap structure that may be the starting point of corrosion is further reduced. In addition to this, the ferritic stainless steel pipe contains 0.001 to 0.600% of Sn in the steel. Therefore, in the unlikely event that corrosion occurs, the eluted Sn ²⁺ ions are adsorbed on the molten surface, and the further elution of the steel base material can be suppressed, and deterioration of the corrosion resistance of the welded portion can be avoided.

Achieving such welds requires a selected shielding gas, especially in welding where shielding gas is required. In particular, there are many gap structures in the thickened portion at the pipe end. That is, the pipe end thickened portion has a structure with many gaps. For this reason, it is preferable to properly shield with an inert gas. Specifically, Ar is most desirable as the shield gas. When mixing CO ₂ or O ₂ with the shield gas, it is desirable that the amount of CO ₂ or O _{2 be} 5% by volume or less.
That is, the method for manufacturing the welded joint of the present embodiment has a step of joining the pipe end thickened portion of the ferritic stainless steel pipe of the present embodiment and the structure by welding. In the joining step by welding, it is preferable to perform welding while supplying a shield gas to the welded portion. Examples of the shield gas include an inert gas such as Ar, a mixed gas of one or both of CO ₂ and O ₂ , and an inert gas. The amount of CO ₂ and O _{2 in} the mixed gas is preferably 5% by volume or less.
Particularly, when the welding method is TIG welding, MIG welding, or mag welding, it is preferable to perform welding while supplying a shield gas to the welded portion. When the welding method is laser welding, it is not necessary to supply the shield gas.

The welded structure of the present embodiment has the welded joint of the present embodiment, and this welded joint further has a structure joined by welding to the pipe end thickened portion, and the single pipe portion of the ferritic stainless steel pipe. Of the welded part, the maximum penetration depth of the ferritic stainless steel pipe side is 0.3t to 2.0t.
According to the ferritic stainless steel pipe satisfying the above composition and the relational expression, it is possible to provide a welded joint and a welded structure having a pipe end thickened portion having excellent salt corrosion resistance in the clearance.
By using this welded joint and welded structure as automobile parts and motorcycle parts in particular, it is possible to reduce the wall thickness of the parts, and it is possible to manufacture efficient parts and improve the fuel efficiency of automobiles and motorcycles to which they are applied. Becomes

In the examples described later, a test was conducted to understand the salt damage resistance of crevices in joints of this type, such as joint A having the structure shown in FIG. 1, and the influence of the elements constituting the ferritic stainless steel pipe and the critical gap distance were examined. Tested.

Hereinafter, the present invention will be described in more detail based on examples.
(Example 1)
Steels having compositions shown in Tables 1 and 2 were melted. In particular, Sn is set to five levels of 0.005, 0.01, 0.03, 0.10% and 0.30% in order to investigate its effect. The molten steel was hot-rolled to a plate thickness of 4 mm, annealed at 1050° C. for 1 minute, and pickled. Then, cold rolling was performed to a plate thickness of 0.8 mm.

Then, 70 mm×70 mm and 40 mm×40 mm test pieces were cut out from the steel plates having the respective compositions shown in Tables 1 and 2, and test pieces having the same component composition were overlapped and spot-welded to form a gap portion of the pipe end thickened pipe. A CCT test piece simulating the above was produced. By adjusting the spot welding conditions, CCT test pieces with various gap intervals were produced.

This CCT test piece was evaluated by the corrosion test method of the appearance of automobile parts of JASO-M610-92. The number of cycles was set to 100 cycles, and after the test, the spot-welded portion was hollowed out to separate the two plates so that the maximum pitting depth in the gap could be evaluated. After the rust was removed, the pitting corrosion depths of the test pieces above and below the gap were measured at 10 points, and the value of the deepest pitting corrosion was taken as the maximum pitting corrosion depth of the steel type. A sample having a maximum pitting depth of less than 500 μm was evaluated as “◯” (good), and a sample having a maximum pitting depth of 500 μm or more was evaluated as “x” (poor).

Table 3 below shows the calculation results of (Cr ² /(1000Sn)) values (Cr and Sn represent the content (% by mass) of each element) of the stainless steel of each composition shown in Tables 1 and ^2. , The value of the gap distance d (μm), the maximum pitting depth (μm) according to the corrosion test method (JASO-M610-92) for the appearance of automobile parts, and the determination result are also shown.

As shown in the graph of FIG. 3, the results shown in Tables 1 to 3 were plotted with the horizontal axis representing the Cr content (mass %) of each sample and the vertical axis representing the gap spacing (d: μm) of each sample. In addition, the Sn amount of each sample is displayed.
In FIG. 3, a chain line with a small interval indicates a curve represented by d=Cr ² /(1000Sn) when the Sn content is 0.10%. Sample No. with Sn content of 0.10%. Sample No. A4, A6, A10, B5, B7, B10. The value of the gap distance d of A4, A10, and A6 is a value above the chain line with the small distance, and the value of Sample No. The value of the gap distance d of B7, B5, and B10 was a value below the chain line with the small distance.
In FIG. 3, the solid line shows a curve represented by d=Cr ² /(1000Sn) when the Sn content is 0.030%. Sample No. with Sn content of 0.030%. Sample Nos. A2, A7, A9, A12, B3, B6, B8, B13, and B14. The value of the gap distance d of A7, A2, A9, A12 is a value above this solid line, and the sample No. The value of the gap distance d of B13, B3, B8, B14, and B6 was a value below this solid line.

In FIG. 3, a chain line having a large interval represents a curve represented by d=Cr ² /(1000Sn) when the Sn content is 0.010%. Sample No. with Sn content of 0.010%. Sample Nos. A1, A3, A13, B1, B2, and B11 were used. The value of the gap distance d of A1, A13, and A3 is a value above the chain line with the large distance, and the sample No. The value of the gap distance d between B1, B2, and B11 was a value below the chain line with the large distance.
In FIG. 3, a thick solid line indicates a curve represented by d=Cr ² /(1000Sn) when the Sn content is 0.005%. Sample No. with Sn content of 0.005%. Sample Nos. A5, A8, A11, B4, B9, and B12 were used. The value of the gap distance d of A5, A8, and A11 is a value above this thick solid line, and the sample No. The value of the gap distance d of B4, B9, and B12 was below the thick solid line.

Sample No. of the present invention example. In each of A1 to A14, the maximum pitting corrosion depth was less than 500 μm, but the sample No. In B1 to B14, the maximum pitting corrosion depth was 500 μm or more.
Therefore, from the results shown in FIG. 3, in the pipe end thickened structure made of the ferritic stainless steel pipe of the present embodiment, the gap distance d (μm) is d≧Cr ² /(1000Sn) (Cr and Sn in the formula). Shows the content (mass %) of each element), it is understood that a pipe end thickened structure having a small maximum pitting depth can be provided.
Further, as shown in FIG. 3, in the ferritic stainless steel pipe according to the present embodiment, it is understood that the pitting corrosion depth in the crevice environment increases as the base material Cr amount increases. Then, in the ferritic stainless steel pipe according to the present embodiment, it is found that the addition of Sn reduces the critical gap distance.

(Example 2)
Steel pipes having a diameter of 60 mm were manufactured by TIG welding using the steel plates having the compositions shown in Tables 1 and 2. By spinning, the end of the steel pipe was folded back inward by 180° to produce a pipe end thickened portion having a length of 50 mm. As described above, a pipe end thickened pipe having a diameter of 60 mm and a length of the end portion (pipe end thickened portion) folded back inward was 50 mm was produced. Then, the pipe end thickened pipe was cut at a length of 60 mm from the folded portion.
In addition, the gap distance of the gap portion in the pipe end thickened portion was set to various values by adjusting the conditions of the spinning process.

A single pipe having a diameter of 62 mm was produced using the same steel plate. A single pipe made of the same steel plate is laid on the outside of the pipe end thickening pipe, and the end (pipe end thickening part) that is folded back inside the pipe end thickening pipe is the welded part. Welding was performed by various methods (TIG welding, MIG welding, mag welding, or laser welding) so that From the above, a CCT test piece having a total length of 100 mm and having a welded portion between the single pipe portion (single pipe pipe) and the pipe end thickened portion at the center was produced.
During various weldings, the amount of current was adjusted to adjust the penetration depth of the weld, and the effect of the penetration depth on corrosion resistance was investigated. In addition, in the case of welding using shielding gas, welding was performed using various shielding gases, and the effect of shielding gas on corrosion resistance was also investigated.
The maximum penetration depth was measured by the following method. Welding was performed under the same conditions, and a CCT test piece was separately prepared. The cross section of the welded portion was observed, and the deepest melted portion in the welded portion was defined as the maximum penetration portion, and the depth was defined as the maximum penetration depth. Specifically, the outer peripheral surface of the end portion of the pipe end thickening pipe (pipe end thickening portion) and the single pipe pipe are overlapped, and the outer peripheral surface side of the end portion of the pipe end thickening pipe (pipe end thickening portion) Welding was performed by bringing the electrode/arc close to. Therefore, the outer peripheral surface of the end portion of the pipe end thickening pipe (pipe end thickening portion) becomes the surface on the electrode/arc side, and the inner peripheral surface of the end portion of the pipe end thickening pipe (pipe end thickening portion) Is the surface (rear surface) opposite to the surface on the electrode/arc side. Pipe end thickening The maximum penetration depth is the distance (depth) from the outer peripheral surface of the end of the pipe (pipe end thickening part) to the maximum penetration part.

This CCT test piece was evaluated by the corrosion test method of the appearance of automobile parts of JASO-M610-92. The number of cycles was set to 100 cycles, and after the test, the welded portion was cut to divide the two plates of the pipe end thickened portion so that the maximum pitting depth in the gap could be evaluated. After the rust was removed, the pitting corrosion depths of the test pieces above and below the gap were measured at 10 points, and the value of the deepest pitting corrosion was taken as the maximum pitting corrosion depth of the steel type. A sample having a maximum pitting depth of less than 500 μm was evaluated as “◯” (good), and a sample having a maximum pitting depth of 500 μm or more was evaluated as “x” (poor).

In Table 4, the penetration depth of the welded portion of the test piece produced using the stainless steel of each composition shown in Tables 1 and 2, the welding shield gas, and the corrosion test method for the external appearance of automobile parts (JASO-M610-92) The maximum pitting depth (μm) and the determination result are also shown.

Assuming that the plate thickness of the single pipe portion of the pipe end thickened pipe is t, from the results of Table 4, when the penetration depth of the welded portion is less than 0.3 t or exceeds 2.0 t, the maximum pitting depth is It can be seen that the thickness is 500 μm or more. It is also found that the maximum pitting corrosion depth is 500 μm or more when the shielding gas at the time of welding contains CO ₂ or O _{2 in} an amount of more than 5% by volume.

According to this embodiment, it is possible to provide a ferritic stainless steel pipe having excellent resistance to salt damage in the gap. Further, by using the steel pipe to which the present embodiment is applied, in particular, as a part for automobiles and motorcycles, it is possible to reduce the thickness, and it is possible to efficiently manufacture parts and improve fuel efficiency.
That is, this embodiment is extremely useful industrially.

A: joint, 1: pipe end thickened structure, 1a: pipe end thickened part, 1b: gap part, 2: steel pipe, 3: welded part.

Claims (8)

C: 0.001 to 0.100% by mass%,
Si: 0.01 to 2.00%,
Mn: 0.01 to 2.00%,
P: 0.001-0.05%,
S: 0.0001 to 0.005%,
Cr: 10.5 to 20.0%,
Sn: 0.001 to 0.600%,
Ti: 0.001-1.000%,
Al: 0.001 to 0.100%,
N: 0.001 to 0.02% is contained, and the balance is Fe and inevitable impurities,
The tube end thickened portion is provided at the tube end, and the gap interval d (μm) formed at the tube end is d≧Cr ² /(1000Sn) (Cr and Sn in the formula are the contents of the respective elements). A ferritic stainless steel pipe having excellent salt damage resistance in the gap, which is characterized by satisfying the relationship of (amount (mass %)). Further, in mass%, Ni: 0.1 to 1.0%,
Mo: 0.1-3.0%,
Cu: 0.10 to 3.00%,
B: 0.0001 to 0.0050%,
Nb: 0.001 to 0.300%,
W: 0.001-1.00%,
V: 0.001 to 0.50%,
Sb: 0.001 to 0.100%,
Co: 0.001 to 0.500%,
The ferritic stainless steel pipe excellent in salt damage resistance of the gap portion according to claim 1, containing any one kind or two kinds or more thereof. Further, in mass% Ca: 0.0001 to 0.0050%,
Mg: 0.0001 to 0.0050%,
Zr: 0.0001 to 0.0300%,
Ga: 0.0001 to 0.0100%,
Ta: 0.001 to 0.050%,
REM: 0.001 to 0.100%,
The ferritic stainless steel pipe having excellent salt damage resistance in the gap according to claim 1 or 2, containing any one or more of the above. The ferritic stainless steel pipe having excellent salt damage resistance in the gap according to any one of claims 1 to 3, which is used for a pipe end thickened structure. A pipe end thickened structure comprising the ferritic stainless steel pipe according to any one of claims 1 to 4. A welded joint having a pipe end thickened portion made of the ferritic stainless steel pipe according to any one of claims 1 to 4. Further having a structure joined by welding to the pipe end thickened portion,
The maximum penetration depth of the welded portion on the side of the ferritic stainless steel pipe is 0.3t to 2.0t, where t is the plate thickness of the single pipe portion of the ferritic stainless steel pipe. The welding joint according to item 6. A welded structure comprising the welded joint according to claim 7.

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